This invention relates to an over-charge prevention circuit, an over-discharge prevention circuit and a charge-discharge control circuit for preventing battery over-charge or over-discharge in a plurality of series connected batteries.
When a plurality of series connected batteries are charged or discharged, variation in the characteristics of each battery presents the problem that one battery may over-charge or over-discharge. Specifically, when one battery of a plurality of series connected batteries has lower capacity than the other batteries, battery charging will continue while the battery unit is not fully charged even though the low capacity battery has reached full charge. As a result, the low capacity battery will become over-charged. Similarly, when the battery unit is in a state that allows discharge, discharge will continue even though the low capacity battery has reached a state where discharge should be suspended. This will subject the low capacity battery to over-discharge.
Japanese Non-examined Patent Publication No.7-163060 issued Jun. 23, 1995 discloses circuitry to prevent over-charge or over-discharge of all batteries of a plurality of series connected batteries and to leave all batteries in the same state of charge or discharge. To avoid over-charging or over-discharging any battery of a plurality of series connected batteries having battery variation, the circuitry cited in this disclosure provides over-charge prevention circuits and over-discharge prevention circuits for each battery. An over-charge prevention circuit is made up of a diode connected in series with the battery, a switching device to bypass charging current, and a voltage detector to control the on-off state of the switching device. An over-discharge prevention circuit is made up of a switching device connected in series with the battery, a diode to bypass discharge current, and a voltage detector to control the on-off state of the switching device.
This circuitry prevents over-charging of a low capacity battery by operation of the over-charge prevention circuit corresponding to that battery to bypass charging current away from that low capacity battery when it becomes fully charged. Other batteries continue to charge to full charge without over-charging.
Similarly during discharge, over-discharge of a low capacity battery is prevented by operation of the corresponding over-discharge prevention circuit to bypass the low capacity battery's discharge current when it has reached a state where discharge should be suspended. Other batteries continue to discharge without over-discharge of any of the batteries.
In this manner, circuitry of the above mentioned disclosure prevents over-charge and over-discharge of all the series connected batteries. However, because an over-charge prevention circuit and an over-discharge prevention circuit are provided for each battery, the number of circuit components is large and the circuit structure is extremely complex.
Further, since the diodes which are components of the over-charge prevention circuits connect in series with the batteries, there is energy loss associated with these diodes. In particular, this energy loss increases as the number of batteries connected in series increases. In addition, the switching devices which are components of the over-discharge prevention circuits and are connected in series with the batteries must be of a type specified to withstand the discharge current.
It is thus an object of the present invention to present a circuit of simple structure which prevents over-charge and over-discharge of a plurality of series connected batteries.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.